Boeing Co. grounded dozens of 737 Max aircraft to repair a new electrical issue that emerged just months after the jets were cleared to return to the skies.

The Chicago-based manufacturer identified the potential problem on some Max jets and alerted 16 customers with the affected serial numbers that they shouldn’t be operated until it is addressed. The company said it’s working with the Federal Aviation Administration on what it described as a “production issue” that affects a specific group of planes, not the entire fleet.

“The recommendation is being made to allow for verification that a sufficient ground path exists for a component of the electrical power system,” Boeing said Friday in a statement.

While the issue isn’t related to the flight-control system that was at the center of the jet’s nearly two-year grounding, it takes the luster off a comeback story that has developed over the first three months of the year. In March, Boeing secured a bumper commitment for the 737 Max from Southwest Airlines Co., which had publicly flirted with Airbus SE’s A220, a smaller single-aisle jet.

Boeing slipped 1% in premarket trading from Thursday’s close of $254.95 in New York.

In addition to the newly revealed production issue afflicting the Max, Boeing has been contending with a separate manufacturing flaw on another model, the 787 Dreamliner. Deliveries of the Dreamliner restarted last month after a five-month drought while the company inspected and repaired tiny wrinkles in the inner lining of the planes’ carbon-fiber barrels.

Impact on Carriers

With the Max jets, the company found the potential problem on a production airplane “during normal build activity,” it said in a message reviewed by Bloomberg.

Boeing declined to say how many of the 183 planes that have been brought into service after the nearly two-year grounding of the model were affected. About 20 operators are conducting about 400 daily flights, according to a separate memo. Those include American Airlines Group Inc., United Airlines Holdings Inc. and Southwest, the largest 737 operator.

It’s premature to estimate how long it will take for the problem to be fixed, said Jessica Kowal, a Boeing spokeswoman. “It could take a matter of hours or a few days,” she said. “We will update you when we have additional information.”

Southwest pulled 30 737 Max 8 from its schedule, according to a statement Friday from the airline, adding that it’s flying only 15 of its 58 Max planes daily and expects “minimal” disruption to its operations. The Dallas-based carrier said it hasn’t experienced any operational problems related to the electrical issue identified by Boeing.

The Max, Boeing’s latest generation of the decades-old single-aisle workhorse, was grounded after two crashes five months apart in Indonesia and Ethiopia in 2018 and 2019, in which 346 people were killed.

It was cleared to fly again by U.S. regulators in November, with Europe and most other major markets following suit.